1. Field of the Invention
The invention relates to a method for operating a multi-roll calender in which a web is guided either through several nips of the calender or through only at least one end nip, depending on a desired surface quality. Furthermore, the invention relates to a calender with a roll stack of several rolls and nips formed between adjacent rolls, in which an end nip limited by an end roll and an intermediate roll is formed by a hard roll and a soft roll, and with a device for holding the intermediate roll limiting the end nip.
2. Discussion of Background Information
Such a method and such a calender are known, e.g., from German Application No. DE 202 00 584 U1. A calender of this type is used mainly to glaze a paper web. With this glazing the paper web is on the one hand condensed, but on the other hand also receives certain surface properties. The aim with the treatment of the paper web in a calender of this type is above all to be able to influence the smoothness of the paper web.
As a first approximation, it can be assumed that, the more nips that are involved in the glazing, the smoothness will be improved further. However, there are also grades of paper in which a high smoothness is not required or not even desired.
Depending on the grade of paper, therefore, either a multi-nip operation is used in which the web is guided through several nips, or a single-nip operating mode in which the web is treated only in a single nip. This nip is then arranged between an end roll and the adjacent intermediate roll. This intermediate roll can be fixed so that with the single-nip operating mode the remaining intermediate roll and the remaining end roll do not have to rotate along with it when “empty”, thus without a web in the nip.
However, the treatment in a single nip in some cases also produces too great a smoothness. In the so-called “matt glazing” the aim is to be able to treat the web between two soft rolls. As a rule, a separate matt calender is necessary for this, which, as an auxiliary unit, incurs expense and requires space.
Supercalenders are also known in which a so-called changeover nip is formed of two soft nips approximately in the middle. If this changeover nip were used for matt glazing, all the rolls would have to be in operation, thus also the upper and lower roll and the other intermediate rolls, although they are not involved in the glazing process. This leads to an unnecessary wear of the surfaces of the rolls and to increased energy consumption. Such an operating mode is thus not desirable as a rule.